A device comprised of two ferromagnetic layers separated by an insulating tunnel barrier is known as a magnetic tunnel junction (MTJ). The cross-section of a conventional MTJ is illustrated in FIG. 1. It consists of bottom terminal 18 placed on a substrate or an underlayer, followed by the first magnetic layer 14, the oxide tunnel barrier 10, the second magnetic layer 12, and the top terminal 16. The resistance of such MTJ is a function of the relative orientation of the magnetizations of the two magnetic layers, being lowest for parallel (P) alignment and highest for anti-parallel (AP) alignment. One of the magnetic layers is made magnetically hard and the other magnetic layer is made magnetically soft such that the two layers switch in different magnetic fields. This can be achieved either by choosing magnetic materials of different magnetic anisotropy or by exchange biasing one of the layers using an antiferromagnetic layer. The P and AP states are realized by applying an external magnetic field sufficient to switch the soft layer but not the hard layer. The associated change in resistance can be used to sense magnetic field in such device as a read head of a hard drive in magnetic recording as described in [1]. Such high-MR MTJ's can also be used as storage elements in MRAM as described in [6]. Here the different resistance values for P and AP states of the junction correspond to stored values of the magnetic bit, “0” and “1”.
It is essential for sensor applications that the signal to noise ratio (SNR) of the device is large. The signal for a magnetic junction sensor is the voltage given by the change in resistance caused by an applied magnetic field (ΔR−deltaR) times the biasing current flowing through the junction (IB), VS=IBΔR. The predominant noise mechanism of the magnetic junction is its thermal noise, which is proportional to the square root of the device resistance, R0.5. In an MTJ the device resistance is dominated by the resistance of the tunnel barrier, such as Al—O, with the leads and the ferromagnetic layers making a vanishingly small contribution. It is therefore highly desirable to decrease the resistance of the barrier (R) without reducing ΔR. R increases as the junction is made smaller in inverse proportion to the junction area (A), the product RA remaining essentially constant. Therefore, for smaller junctions of the future generations of field sensors the demand for reducing the resistance of the barrier is even higher. The desired values for the resistance-area product are RA<1000 Ohm-um2, preferably RA<10 Ohm-um2 as described in [2]. Similar noise and scaling considerations apply to MTJ's used in MRAM [6]. Additionally, low resistance barriers are required for MRAM cells utilizing the effect of current induced magnetization switching, described in [3,4] and experimentally observed in [5]. Relatively high currents (typically a few mA) flowing through an MTJ are needed for producing switching of the soft magnetic layer. In order not to exceed the breakdown voltage of the MTJ (approximately 1 V) the resistance of the junction should preferably be smaller than 1 kOhm. This corresponds to preferably RA<10 Ohm-um2 for a junction of ˜0.01 um2 in area.
The resistance of a tunnel barrier is an exponential function of the barrier thickness, with thinner barriers having lower resistance. For very thin barriers of thickness less then 1 nm, however, the useful signal (ΔR) decreases due to microscopic pinholes and other defects in the barrier as described in [2]. It is therefore desirable for sensor and MRAM applications to have an independent means of reducing the resistance of the barrier while preserving the barrier thickness, necessary for achieving high MR.
Another desirable characteristic of a magnetic junction is current rectification. This property is conventionally realized in semiconductor diodes and is manifest in different currents through the device for a given bias voltage of positive and negative polarity. The ratio of these two current values is known as the rectification ratio (RR). Incorporating a diode in series with an MTJ in an MRAM cell can provide a significantly improved memory density as described in [6]. The diode blocks current for one bias polarity through MTJ's placed at cross-points of a 2D array of word and bit lines and allows current for the other bias polarity, thus providing a cell select mechanism built into the MTJ stack. However, fabricating efficient semiconductor diodes within metal-oxide MTJ stacks is a highly non-trivial task. It is highly desirable that the diode function is implemented within the same metal-oxide material system. Double tunnel junctions of the general structure magnet1/oxide/metal/oxide/magnet2 with atomically thin metal center electrodes and asymmetric oxide tunnel barriers can exhibit current rectification as described in [7]. Such diode structure is a variation of a conventional semiconductor Resonant Tunneling Diode described in detail in e.g. [8]. The spin sensitivity of the outer electrodes (magnet1 and magnet2) combined with transport through discrete energy states in the center electrode can provide high-MR and diode functionality—an ideal combination for use in MRAM applications. The thickness of the center metal layer must be comparable to its Fermi wavelength, which is smaller than 1 nm in most metals. For such thin metal layers dimensional quantization in the direction of current (perpendicular to the layers) results in transport through discrete electron states in the layer. Roughness of even one monolayer can significantly affect the performance of the device. The need for atomically thin and atomically smooth metal layers imposes practical limitations on the use of such a double barrier device where the center electrode is a metal.
The electronic energy states of the center electrode in the double-junction described above can be affected electrically by an additional electrode placed in physical or electrical contact to the center electrode—a gate. Such a spin-dependent three-terminal device is a variation of a conventional Resonant Tunneling Transistor described in detail in e.g. [9]. The combination of MR and gate control can provides the basis for novel logic applications, in particular in reprogrammable logic [10].
Use of MgO yields MR (ΔR/R) of 100%-1000% in magnet1/MgO/magnet2 type junctions as described in [11]. The commonly used materials for the magnet layers are Fe, Co, Fe—Co, Fe—Ni, Fe—Co—B alloys. Magnetic junctions based on MgO are preferred in applications based on MR compared to junctions based on, for example, Al and Ti oxides where the typical MR value is ˜30%. Therefore, it is highly desirable for sensor, memory, and logic applications described above to improve the MgO barrier in such a way as to lower its resistance for a given thickness.
Thus, it is in the art demonstrated the envisaged potentials of devices utilizing tunneling effects, comprising dual magnetic layers. However, the performance of such devices is presently often impaired by defects relating to the problems of providing thin enough layers separating the magnetic layers.